The purpose of this project is to test and define experimentally an immunoregulatory mechanism that is based upon a self-regulating network of antigen-specific antibody-forming cell clones (clonotypes) and autologous idiotype-specific antibody-forming cell clones (anticlonotypes). After fusing immune spleen cells with a mutant myeloma cell line, we are able to identify myeloma/spleen cell hybrids secreting monoclonal antibodies specific for the immunizing ligands (either a thymic-independent, thymic-dependent or tumor-specific antigen) and myeloma/spleen cell hybrids secreting antibodies specific for autologous idiotypes on the antigen-specific antibodies. By employing a rapid screening procedure with the cell-fusing technique we can select, clone and cultivate hybrids secreting antigen-specific and idiotype-specific antibodies from numerous immune spleen cell populations. The ability of some resultant hybrids to secrete idiotype-specific antibodies presumably reflects the generation of idiotype reactive cells under physiological conditions, which is consistent with the proposed network theory. The selective elimination of the idiotype-reactive (anti-idiotypic) cells was shown to have two significant effects. Immunization of spleen cells depleted of the idiotype-reactive cells resulted in (1)\a clonal dominance of the antigen-specific clonotype bearing the reference idiotype, and (2)\the expression of an idiotypepositive clonotype(s) which is (are) not specific for the antigen. The study of the mechanisms of idiotype-specific regulation of B-cell clonotypes are being pursued in vivo and in vitro.